The present invention relates to the measurement of electrical signals and, in particular, to bias tees used therein.
The use of bias tees in measurements is well-known. Referring to FIG. 1, a prior art bias tee 10 is used to connect a DC instrument 12. (e.g, a source measurement unit) and a high frequency (HF) instrument 14 to a device under test 16 (DUT). The HF instrument 14 may be, for example, a combination RF signal generator and measurement device. It should be noted that the bias tee 10 can also be used in the case of pulses as opposed to radio frequency signals per se, the frequency content of typical interest being similar.
The typical purpose of the bias tee 12 is to allow the simultaneous application of HF and low frequency (e.g., DC) signals to the DUT 16. At the same time, DC signals are blocked from the instrument 14 and HF signals are blocked from the instrument 12. For example, the DC signals may serve to bias the DUT while it responds to RF signals, hence the name, bias tee.
The example of FIG. 1 includes both a source path 18 and a measure path 20, so that a so-called Kelvin connection can be made to the DUT to improve measurement performance with respect to the low frequency signals.
Unfortunately, the bias tee 10 introduces its own measurement errors, for example, leakage and stray capacitances. This particularly creates challenges in making very accurate and/or very sensitive measurements (e.g., pico- and femto-ampere measurements).